1. Field of the Invention
The present invention relates to methods and apparatus utilized in conducting continuous chemical and biological reactions. More particularly, the present invention is directed to construction and use of reactors (such as bioreactors) for use in connection with chemical and biological reactions.
2. The Prior Art
As a result of the dramatic increase in knowledge concerning biological reactions in recent years, and also because of vast improvements in techniques for isolating particular enzymatic compounds, there has developed a substantial industrial reliance upon biochemical reactions to obtain desired biochemical products.
Reactions involving biological materials are typically conducted in reaction vessels generally known as "bioreactors." The simplest type of bioreactor is nothing more than a vat into which is introduced the biological material, such as a microorganism or enzyme, together with appropriate reactants and/or nutrients. After an appropriate period of time, the products are separated from the biological material such as by use of a polymer membrane filter, and the biological material is returned to the bioreactor.
It will be appreciated that use of a vat as a bioreactor results in a batch process, thereby yielding an intermittent production of products. Also, because reaction products that form early in the batch cycle are retained in the vat during the entire batch cycle, it is not uncommon for desired products to degrade or to undergo additional undesirable reactions. Further, this batch type of bioreactor allows for very little control over reaction parameters other than gross parameters such as temperature.
Although a vat-type bioreactor is adequate for some biological reactions, because of the foregoing problems, many reactions are performed in more recently-developed bioreactors which comprise a column filled with glass beads. In such a "packed column" bioreactor, the glass beads are first treated so that the surface of the beads are coated with a thin film of an organic substance which will attract and adhere to the biological material to be reacted; various silanes and olefins are often used for this purpose. A solution containing the biological material is then fed through the column so that the biological material will become affixed to the organic coating on the glass beads and thus be held in the column. Finally, a solution containing reactants is introduced into the column, and products and unreacted reactants are collected at the bottom of the column.
One important advantage of the packed column-type bioreactor over a vat-type bioreactor is that the packed column-type is a semi-continuous system that provides a more relatively constant supply of reaction products than obtained in vat-type bioreactors. Another advantage of the packed column-type bioreactors is that the size of the glass beads and the solution flow rate can be adjusted in order to control the reaction occurring within the column. Further, since the reaction products are continually removed from the system, the likelihood of occurrence of degradation or unwanted reactions is significantly reduced as compared to the vat-type bioreactor.
However, while much better than a vat-type bioreactor, since the reaction rate is primarily diffusion limited, a packed column-type bioreactor still provides for little control of many reaction parameters, particularly at the reaction site. Also, since only the surfaces of the beads have biological material attached thereto, there is much unused space within the column; hence, it is typically necessary to use very large columns. In addition, although products and unused reactants are continually removed from the column system, thereby significantly reducing the amount of unwanted side reactions as compared to such reactions in a vat-type bioreactor, some product is generated near the entry point of the column, and must thus negotiate substantially the entire length of the column before being removed from the system. Thus, the potential for significant degradation and unwanted side reactions still remains. One further problem when utilizing columns is the difficulty in cleaning and sterilizing the column and beads between uses; it is a burdensome and time-consuming task to unpack, clean, and repack a column between uses.
Another recently developed reactor involves the use of thin microporous sheets of a polyvinylchloride-silica (PVC-silica) material that are placed within the reactor so that reaction solution flows through the microporous sheets. Various chemicals and enzymes are capable of binding to active sites (attributed to the presence of silica) within the porous matrix of these sheets.
However, the use of PVC-silica sheets in a bioreactor also suffers from significant disadvantages. For instance, the rate of diffusion of fluid across a microporous PVC-silica membrane is inversely proportional to the thickness of the membrane. Thus, in order to accommodate high volume throughput, it is extremely advantageous to provide a very thin membrane. However, rarely will acceptable volumes of fluid diffuse through even a very thin membrane without application of pressure as a driving force. Typically, it is necessary to apply substantial pressure in order to drive acceptable volumes of fluid through such membranes.
Unfortunately, the use of high pressures across a very thin PVC-silica membrane results in substantial stress on the membrane, typically resulting in degradation or deformation of the membrane. Deformation causes the pore sizes to become effectively larger in some places, thereby permitting oversized fluid components to pass therethrough, and undersized in other locations, thereby filtering out fluid components that should be permitted to pass through the membrane. Frequently, the thickness of the membrane must be substantially increased so that the membrane will be capable of withstanding the considerable stress imparted by the pressurized fluid. This, of course, is somewhat self-defeating since the act of thickening the membrane acts to reduce the volume throughput, thereby mandating even greater pressures. As a result, a less than optimum compromise must be made with respect to the pressures used to drive fluid through the membrane, and the thickness of the membrane.
Membranes formed from materials such as polysulfone and polypropylene have also been utilized as matrix material in bioreactors. In addition to problems similar to those discussed above, bioreactors utilizing membranes formed from these materials have exhibited problems referred to as "cell release" due to the deformability of the material under pressure that builds up due simply to cell growth.
Another porous membrane-type bioreactor that has been utilized is provided with a membrane constructed from ceramic rather than from a PVC-silica material. One advantage of using a ceramic membrane is that it is less subject to deformation than a PVC-silica membrane or a membrane constructed from a polysulfone or polypropylene material. However, a ceramic membrane remains subject to degradation under high pressures if the membrane is too thin; hence, it remains necessary to compromise between the pressures used to drive fluid through the membrane, and the thickness of the membrane.
A further difficulty encountered when using membrane sheets is that due to fluid dynamics, flow through the center of the sheet will be significantly faster than flow through the sheet near the edges. As a result, when the sheet is used as a support matrix for a chemical or biological material to be used in effecting a desired reaction, the flow of reaction solution past the chemical or biological material will be much greater in some portions of the membrane than in others. Since the flow rate is an important determinant of reaction rate and of the efficiency of the reaction, this difference in flow rate from location to location on the membrane can have a substantial adverse effect on the efficiency of the reaction system.
One further difficulty sometimes encountered when using microporous membranes is that the use of high pressures can sometimes be detrimental to various fluid components. For instance, high pressures can be extremely detrimental to some cellular components that might be used to effect biological reactions. Also, use of high pressures might cause chemical or biological materials fixed within the pores of material to break loose and be carried away in the reaction solution. Thus, in some cases, the use of a typical PVC-silica membrane or a ceramic membrane requires use of relatively low pressures, with a correspondingly low volume throughput. However, because of the flexibility of PVC-silica material, it is sometimes not feasible to provide PVC-silica membranes thin enough to permit adequate flow rates when using such low pressures, and yet capable of maintaining a controlled pore size.
In view of the foregoing, it will be appreciated that it would be a substantial advancement if an improved bioreactor could be provided that was capable of increased control over reaction parameters. It would also be a significant advancement if a new bioreactor were provided that had the capability to collect reaction products separately from reactants and unwanted by-products, and that could minimize unwanted side reactions. Additionally, it would be a significant advancement if a new bioreactor were to be provided that could be readily cleaned and sterilized for reuse. Such a bioreactor is described and claimed herein.